1.2 ELT concepts under consideration

Amongst current proposals for a leap beyond the 8-m class of ground-based
telescopes, the least expensive-looking are those originating from the
team associated with the ~9.3m Hobby-Eberly Telescope (HET) for a 25-m
using the same design principles (Bash et al. 1996 and references
therein). The HET is a minimum-cost VLT currently being commissioned in
Texas and likely soon to be cloned in South Africa. It and its proposed
ELT offspring use a design resembling that of the Arecibo radio telescope,
with the segmented, spherical primary mirror tilted at a fixed zenith angle
and therefore subject to a constant gravity vector. Rotation is in azimuth
only and is employed only for acquisition: a mobile focal plane feed assembly,
providing spherical aberration correction, tracks targets for 45 minutes
or more as the entrance pupil moves across the stationary primary mirror.
The Hobby-Eberly Telescope was built (though not yet commissioned) for
the remarkably low cost of ~$14M, and the team estimate the total cost
of their scaled-up 25-m ELT version at around $250M (Sebring et al.1998)
.

The builders of the highly successful Keck telescopes are currently
studying proposals for a 30-m successor. Initial concepts were presented
by Nelson & Mast (1999) and a more recent outline of the design concepts
was given by Nelson (2000). This differs from the HET-clone and OWL (see
below) proposals in several ways, but in particular in adopting a non-spherical
primary for a Ritchey-Chretien design, the proposers believing that the
complications of fabricating large numbers of aspheric elements can be
overcome inside an attainable budget (c.f. Mast, Nelson & Sommargren,
2000). This proposal explicitly includes the capability to deliver a large
(~20 arcmin) FOV in order to permit seeing-limited (~0."5 resolution) multi-object
spectroscopy as well as near-diffratcion-limited MCAO operation.

The arguments outlined by Mountain (1996) have tended to drive a number
of studies in the direction of a facility significantly larger then the
25-m to 30-m concepts outlined above. Both the Swedish group (c.f. Ardeberg
et
al., 1996: 25 m; Ardeberg, Andersen & Owner-Petersen, 1998: 25-50
m; Gontcharov & Owner-Petersen, 2000: 50 m) and the US-based studies
of MAXAT (Maximum-Aperture Telescope) have been following this trend (though
the recent emphatic specification of the Giant Segmented Mirror Telescope
(GSMT) as a
30-m facilityby the Decadal Survey may have reversed it).
This is perhaps a pity, as the science case for ELTs grows ever more attractive
with aperture; indeed the 50-m proposals should perhaps be seen as 100-m
concepts (see below) constrained by fear of the technical challenges. Even
so, 50-m and 100-m propjects have many concerns in common and much synergy
between the groups continues.

As noted above, the ambitious proposal for a 100-m facility (OWL) was
first put forward by Gilmozzi
et
al. in 1998. In recent incarnations OWL has evolved through a number
of possible optical configurations, most assuming a 100-m spherical primary
mirror and a 15- to 25-m spherical (Gilmozzi
et al.1998) or flat
segmented secondary (e.g. Brunetto, Koch & Quattri, 1999). Recent
developments include a comprehensive examination of optical design and
practicality issues for such a facility and its instrumentation at the
UK Astronomy Technology Centre (Atad-Ettedgui et al., 2000). This
work has highlighted some of the difficulties in providing a realistic
``seeing-limited" capability for so large a telescope, amongst other instrumental
issues, and also the potentially important problem of the degrading effects
of atmospheric dispersion, even in the NIR, for image sizes of a few milli-arcsec.
An active study group under the aegis of the European Southern Observatory
(ESO) (c.f.
Dierickx & Gilmozzi, 2000) is continuing the earlier design work.

The telescope is currently envisaged as being partly recessed into the
ground, with the primary mirror a little above that level, so that much
of the supporting structure is below ground level when observing near the
zenith. The telescope would have the ability to tilt its optical axis down
to the horizontal to allow engineering on the topend and to permit it to
be protected in a roll-away structure when not in use. Informal cost estimates
for this facility suggest that it can be built for of order $10e9; but
as noted, scepticism about such estimates has in part been responsible
for the concentration of other groups on 50-m alternatives.

That said, the vigorous studies under way in Europe are firmly based
on demonstrated technololgies and are explicitly oriented towards industrial
(mass) production, not only of the optics (see below, section 1.3
(2)) but also of the mechanical structure. Much of the final forming of
the components of the latter is envisaged as occurring on site and even
in
situ ("self machining").